Image sensor, image-capturing apparatus, and electronic device

ABSTRACT

An image sensor includes: a readout circuit that reads out a signal to a signal line, the signal being generated by an electric charge resulting from a photoelectric conversion; a holding circuit that holds a voltage based on an electric current from a power supply circuit; and an electric current source including a transistor having a drain part connected to the signal line and a gate part connected to the holding circuit and the drain part, the electric current source supplying the signal line with an electric current generated by the voltage held in the holding circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of application Ser. No. 17/695,345 filed Mar. 15,2022, which in turn is a Continuation of application Ser. No. 16/590,719filed Oct. 2, 2019, which is a Continuation of application Ser. No.15/764,066 filed Mar. 28, 2018 (now U.S. Pat. No. 10,554,916), which isa National Stage of International Patent Application No.PCT/JP2016/078478 filed on Sep. 27, 2016, which claims the benefit ofJapanese Patent Application No. 2015-195279 filed Sep. 30, 2015. Thedisclosure of the prior applications is hereby incorporated by referenceherein in its entirety.

TECHNICAL FIELD

The present invention relates to an image sensors, an image-capturingapparatus sensor, and an electronic device.

BACKGROUND ART

An image-capturing apparatus has been known which can perform parallelprocessing on a signal read from a pixel for individual unit pixel cellsor for individual cells including a plurality of pixels (PTL1).

CITATION LIST Patent Literature

-   PTL1: Japanese Laid-Open Patent Publication No. 2012-244331

SUMMARY OF INVENTION

According to the 1st aspect of the present invention, an image sensorcomprises: a readout circuit that reads out a signal to a signal line,the signal being generated by an electric charge resulting from aphotoelectric conversion; a holding circuit that holds a voltage basedon an electric current from a power supply circuit; and an electriccurrent source including a transistor having a drain part connected tothe signal line and a gate part connected to the holding circuit and thedrain part, the electric current source supplying the signal line withan electric current generated by the voltage held in the holdingcircuit.

According to the 2nd aspect of the present invention, an image sensorcomprises: a first readout circuit that reads out a first signal to afirst signal line, the first signal being generated by an electriccharge resulting from a photoelectric conversion; a second readoutcircuit that reads out a second signal to a second signal line, thesecond signal being generated by an electric charge resulting from aphotoelectric conversion; a first holding circuit that holds a voltagebased on an electric current from a power supply circuit; a secondholding circuit that holds a voltage based on the electric current fromthe power supply circuit; a first electric current source including afirst transistor having a first drain part connected to the first signalline and a first gate part connected to the first holding circuit andthe first drain part, the first electric current source supplying thefirst signal line with an electric current generated by the voltage heldin the first holding circuit; and a second electric current sourceincluding a second transistor having a second drain part connected tothe second signal line and a second gate part connected to the secondholding circuit and the second drain part, the second electric currentsource supplying the second signal line with an electric currentgenerated by the voltage held in the second holding circuit.

According to the 3rd aspect of the present invention, an electronicdevice, comprises: an electronic circuit having a plurality ofelectronic elements; a holding circuit that holds a voltage based on anelectric current from a power supply circuit; and an electric currentsource including a transistor having a drain part connected to theelectronic circuit and a gate part connected to the holding circuit andthe drain part, the electric current source supplying the electroniccircuit with an electric current generated by the voltage held in theholding circuit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of animage-capturing apparatus 1 according to a first embodiment.

FIG. 2 is a circuit diagram illustrating a configuration of a pixel 10according to the first embodiment.

FIG. 3 is a circuit diagram illustrating a configuration of an electriccurrent source 30 and a storage circuit 110 according to the firstembodiment.

FIG. 4 is a timing chart illustrating an exemplary operation of acontrol unit 33 according to the first embodiment.

FIG. 5 is a circuit diagram illustrating an exemplary application of theelectric current source 30 according to the first embodiment.

FIG. 6 is a circuit diagram showing another exemplary application of theelectric current source 30 according to the first embodiment.

FIG. 7(a) is a circuit diagram illustrating a part of a pixel circuit150, an electric current source circuit 50, and a reference electriccurrent source circuit 31 according to a second embodiment. FIG. 7(b) isa diagram illustrating a connection relationship between a part of thepixel circuit 150, the electric current source circuit 50, and thereference electric current source circuit 31 according to the secondembodiment.

FIG. 8 is a circuit diagram illustrating a configuration of an electriccurrent source 30 and a storage circuit 110 according to the thirdembodiment.

FIG. 9(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to afirst variation. FIGS. 9(b) and 9(c) are timing charts illustrating anexemplary operation of the electric current source circuit 50 and itsperipheral circuits according to the first variation.

FIG. 10(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to asecond variation. FIGS. 10(b) and 10(c) are timing charts illustratingan exemplary operation of the electric current source circuit 50 and itsperipheral circuits according to the second variation.

FIG. 11(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to athird variation. FIG. 11(b) is a timing chart illustrating an exemplaryoperation of the electric current source circuit 50 and its peripheralcircuits according to the third variation.

FIG. 12(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to afourth variation. FIG. 12(b) is a diagram illustrating a configurationexample of a part of the pixel 10 according to the fourth variation.FIG. 12(c) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the fourth variation.

FIGS. 13(a) to 13(d) are diagrams illustrating a configuration of anelectric current source 30 and its peripheral circuits according to afifth variation.

FIGS. 14(a) to 14(c) are timing charts illustrating an exemplaryoperation of the electric current source 30 and its peripheral circuitsaccording to the fifth variation.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is a block diagram illustrating a configuration of animage-capturing apparatus 1 according to a first embodiment. Theimage-capturing apparatus 1 includes an optical system 2, an imagesensor 3, and a control unit 4. The optical system 2 emits light from asubject to the image sensor 3. The image sensor 3 captures an image ofthe light emitted from the optical system 2 to generate image data, forexample. The control unit 4 performs various image processes on theimage data outputted from the image sensor 3. Additionally, the controlunit 4 outputs a control signal for controlling an operation of theimage sensor 3 to the image sensor 3. It should be noted that theoptical system 2 may be detachably mounted in the imaging apparatus 1.

FIG. 2 is a circuit diagram illustrating a configuration of a pixel 10according to the first embodiment. The image sensor 3 has a plurality ofpixels 10. The pixel 10 includes a photoelectric conversion unit 12 anda readout circuit 100. The photoelectric conversion unit 12 is arranged,for example, in a matrix in an image-capturing region of the imagesensor 3. The photoelectric conversion unit 12 has a photoelectricconversion function of converting incident light into an electriccharge. The photoelectric conversion unit 12 accumulates the electriccharge resulting from the photoelectric conversion. The photoelectricconversion unit 12 is composed of a photodiode, for example. The readoutcircuit 100 reads out a pixel signal to a signal line 17, the pixelsignal being generated by the electric charge resulting from thephotoelectric conversion performed by the photoelectric conversion unit12. The pixel signal represents image data, for example. The readoutcircuit 100 includes a transfer unit 13, a discharge unit 14, a floatingdiffusion 15, and an output unit 16.

The transfer unit 13 transfers the electric charge resulting from thephotoelectric conversion performed by the photoelectric conversion unit12 to the floating diffusion 15. In other words, the transfer unit 13forms an electric charge transfer path between the photoelectricconversion unit 12 and the floating diffusion 15. The output unit 16outputs a pixel signal to the signal line 17, the pixel signal beinggenerated by the electric charge transferred by the transfer unit 13from the photoelectric conversion unit 12 to the floating diffusion 15.The output unit 16 is a transistor having a drain terminal, a gateterminal, and a source terminal that are respectively connected to apower supply VDD, the floating diffusion 15, and the signal line 17. Thedischarge unit 14 discharges the electric charge in the floatingdiffusion 15. The floating diffusion 15 is reset to a referencepotential as a result of the discharge of the electric charge by thedischarge unit 14.

An electric current source 30 is connected to the readout circuit 100via the signal line 17. The electric current source 30 supplies anelectric current for causing the readout circuit 100 to read out thepixel signal generated by the electric charge resulting from thephotoelectric conversion performed by the photoelectric conversion unit12. Specifically, the electric current source 30 is a transistor havinga drain terminal, a gate terminal, and a source terminal that arerespectively connected to the signal line 17, a reference electriccurrent source circuit 31, and a ground (GND). The electric currentsource 30 supplies an electric current to the output unit 16 of thereadout circuit 100. In other words, the output unit 16 forms a sourcefollower circuit with the electric current source 30 as a load electriccurrent source. The electric current source 30 generates the electriccurrent to be supplied to the signal line 17, based on an electriccurrent from the reference electric current source circuit 31.Additionally, the electric current source 30 has a drain terminal and agate terminal that are connected to each other via a switch.

FIG. 3 is a circuit diagram illustrating a configuration of the electriccurrent sources 30 according to the first embodiment and storagecircuits (also referred to as holding circuits) 110 for generating anelectric current to be supplied to the signal line 17 by the electriccurrent source 30. The example in FIG. 3 illustrates only three currentsources 30 (electric current sources 30A to 30C) for the sake ofsimplification of description.

The reference electric current source circuit 31 has a referenceelectric current source I1. The reference electric current source I1 isconnected to the power supply VDD and outputs a reference electriccurrent i1. The electric current source 30 supplies the signal line 17with an electric current corresponding to the reference electric currenti1 outputted from the reference electric current source I1.

The storage circuit 110 has a storage unit 32 and a control unit 33. Thestorage circuit 110 (each storage circuit 110A to 110C) stores a voltagebased on the reference electric current i1 outputted from the referenceelectric current source I1. The storage unit 32 of the storage circuit110 stores (holds) the voltage based on the reference electric currenti1 outputted from the reference electric current source I1. The storageunit 32 is connected to the gate terminal of the electric current source30 and supplies the electric current source 30 with the voltage stored(held) in the storage unit 32.

The electric current source 30 supplies the signal line 17 with anelectric current based on the voltage stored in the storage unit 32. Thestorage unit 32 is constituted of a capacitive element such as acapacitor having one electrode connected to the gate terminal of theelectric current source 30 and the other electrode connected to theground, for example. In the example illustrated in FIG. 3 , the storageunits 32 (i.e., the storage units 32A to 32C) are respectively composedof capacitors C1 to C3.

The control unit 33 controls the electric current supplied from thereference electric current source circuit 31 to the storage unit 32. Forexample, when the voltage based on the electric current outputted fromthe reference electric current source I1 is to be stored in the storageunit 32A, the control unit 33 controls electric currents supplied fromthe reference electric current source I1 to the storage unit 32B and thestorage unit 32C to be smaller than the electric current supplied fromthe reference electric current source I1 to the storage unit 32A.

The control unit 33 includes switches that connect the referenceelectric current source circuit 31 and the storage unit 32, for example.Each control unit 33 (the control unit 33A to 33C) has a switch SWS(SWS1 to SWS3, respectively), a switch SWD (SWD1 to SWD3, respectively),and a switch SWO (SWO1 to SWO3, respectively).

Each of the switches SWS (SWS1 to SWS3), SWD (SWD1 to SWD3), and SWO(SWO1 to SWO3) is composed of a transistor, for example. The switchesSWS (SWS1 to SWS3), SWD (SWD1 to SWD3), and SWO (SWO1 to SWO3) arecontrolled by a control signal outputted by a control circuit (notshown). It should be noted that resistors R1 to R3 are wiring resistorsof a wiring connected to the ground.

FIG. 4 is a timing chart illustrating an exemplary operation of thecontrol unit 33 according to the first embodiment. In FIG. 4 , symbolsSWS1 to SWS3, SWD1 to SWD3, and SWO1 to SHO3 denotes control signalsinputted from the control circuit to the switches SWS (SWS1 to SWS3),SWD (SWD1 to SWD3), and SWO (SWO1 to SHO3). Additionally, the verticalaxis represents voltage level of the control signals outputted from thecontrol circuit (not shown), and the horizontal axis represents time.The switches SWS (SWS1 to SWS3), SWD (SWD1 to SWD3), and SWO (SWO1 toSHO3) are turned on if the input control signal is at a high level andturned off if the input control signal is at a low level.

At a time t1, the switches SWS1 and SWD1 are turned to high level. Oncethe switch SWD1 is turned on, the gate terminal and the drain terminalof the electric current source 30A are connected to each other. As aresult, the electric current source 30A is diode-connected. Furthermore,turning on the switch SWS1 connects the reference electric currentsource I1 with the electric current source 30A and the storage unit 32A.This causes the reference electric current i1 to be supplied from thereference electric current source I1 to the electric current source 30Aand the storage unit 32A.

Agate-source voltage Vgs of the electric current source 30A is a valuedepending on the reference electric current i1 and a threshold voltageVth1 of the electric current source 30A. The voltage Vg1 applied to thegate terminal of the electric current source 30A is a value depending onthe gate-source voltage Vgs of the electric current source 30A and is avoltage based on the reference electric current i1 and the thresholdvoltage Vth1 of the electric current source 30A. The voltage Vg1 basedon the reference electric current i1 is stored in the storage unit 32A.

At a time t2, the switch SWD1 is turned to low level. Once the switchSWD1 is turned off, the reference electric current source I1 and thestorage unit 32A are disconnected. The gate-source voltage Vgs of theelectric current source 30A is stored in the storage unit 32A. Turningoff the switch SWD1 prior to the switch SWS1 can prevent the voltage Vg1stored in the storage unit 32A from dropping through the electriccurrent source 30A.

At a time t3, the switch SWS1 is turned to low level. Once the switchSWS1 is turned off, the reference electric current source I1 and thestorage unit 30A are disconnected. Likewise, turning on the switch SWS2and the switch SWD2 connects the reference electric current source I1with the electric current source 30B and the storage unit 32B. Thiscauses the reference electric current i1 to be supplied from thereference electric current source I1 to the electric current source 30Band the storage unit 32B. In a period from a time t4 to a time t5, theswitch SWD2 and the switch SWS2 are turned to low level. Once the switchSWD2 and the switch SWS2 is turned off, the reference electric currentsource I1 and the storage unit 32B are disconnected. The gate-sourcevoltage Vgs of the electric current source 30B is stored in the storageunit 32B.

At a time t5, the switches SWS3 and SWD3 are turned to high level.Turning on the switch SWS3 and the switch SWD3 connects the referenceelectric current source I1 with the electric current source 30C and thestorage unit 32C. This causes the reference electric current i1 to besupplied from the reference electric current source I1 to the electriccurrent source 30C and the storage unit 32C. In a period from a time t6to a time t7, the switches SWD3 and SWS3 are turned to low level. Oncethe switches SWD3 and SWS3 are turned off, the reference electriccurrent source I1 and the storage unit 32C are disconnected. Thegate-source voltage Vgs of the electric current source 30C is stored inthe storage unit 32C.

At a time t8, the switches SWO1 to SWO3 are turned to high level. Oncethe switches SWO1 to SWO3 are turned on, the electric current sources30A to 30C supply the signal lines 17A to 17C with electric currentsbased on the voltages Vg1 to Vg3 applied to the respective gateterminals.

FIG. 5 is a circuit diagram illustrating an exemplary application of theelectric current source 30 according to the first embodiment. Theelectric current sources 30A to 30C supply electric currents to theoutput units 16 of the respective readout circuits 100A to 100C via thesignal lines 17.

The electric current sources 30A to 30C are supplied with the referenceelectric current from the reference electric current source I1 by thecontrol units 33A to 33C. The electric current sources 30A to 30C aresupplied with the reference electric current i1 in the order of theelectric current source 30A, the electric current source 30B, and theelectric current source 30C by means of the switches SWS1 to SWS3, SWD1to SWD3, and SWO1 to SWO3 of the control units 33A to 33C. As a result,voltages based on the reference electric current i1 from the referenceelectric current source I1 are stored in the storage units 32A to 32C.In other words, the voltages Vg1 to Vg3 based on the reference electriccurrent i1 and the threshold voltages Vth1 to Vth3 of the electriccurrent sources 30A to 30C are stored in the storage units 32A to 32C,respectively.

The electric current sources 30A to 30C supply the signal lines 17A to17C with electric currents generated by the voltages based on thereference electric current i1 from the reference electric current sourceI1 stored in the storage units 32A to 32C, respectively. In other words,the electric current sources 30A to 30C supply the respective outputunits 16 of the readout circuits 100A to 100C with electric currentsgenerated by the voltages Vg1 to Vg3 based on the reference electriccurrent i1 from the reference electric current source I1 stored in thestorage units 32A to 32C, respectively.

FIG. 6 is a circuit diagram illustrating another exemplary applicationof the electric current source 30 according to the first embodiment.Although FIG. 5 illustrates the exemplary application in which theelectric current sources 30A to 30C supply electric currents to thecorresponding readout circuits 100, the present invention is not limitedto this. FIG. 6 illustrates an exemplary application in electric currentsources of comparator circuits 21A to 21C constituting a part of ananalog/digital conversion circuit that is connected to the signal lines17A to 17C illustrated in FIG. 5 and converts the pixel signals read outby the readout circuit 100 into digital signals.

The electric current sources 30A to 30C supply the respective comparatorcircuits 21A to 21C with electric currents. In the example illustratedin FIG. 6 , the electric current sources 30A to 30C are composed of PMOStransistors M10 to M30. The comparator circuit 21 includes transistorsM6 to M9, for example. The transistors M6 and M7 have their sourceterminals commonly connected to form a differential pair. Thetransistors M8 and M9 function as an active load unit. A pixel signal isinputted to the gate terminal of one of the transistors M6 and M7 fromthe signal line 17A to 17C illustrated in FIG. 5 directly or via acapacitor or other elements, and a reference signal is inputted to thegate terminal of the other of the transistors M6 and M7 directly or viaa capacitor or other elements. The electric current sources 30A to 30Csupply source terminals of the transistors M6 and M7 with electriccurrents and thus function as tail electric current sources of thecomparator circuits 21.

The comparator circuit 21 outputs an output signal generated bycomparing the pixel signal and the reference signal, to a latch circuit.Based on the output signal of the comparator circuit 21, the latchcircuit holds a count value as a function of the time elapsed since thestart of the comparison.

The electric current sources 30A to 30C are supplied with the referenceelectric current from the reference electric current source I1 by thecontrol units 33A to 33C. The electric current sources 30A to 30C aresupplied with the reference electric current i1 in the order of theelectric current source 30A, the electric current source 30B, and theelectric current source 30C by means of the switches SWS1 to SWS3, theswitches SWD1 to SWD3, and the switches SWO1 to SWO3 of the controlunits 33A to 33C. As a result, voltages based on the reference electriccurrent i1 from the reference electric current source I1 are stored inthe storage units 32A to 32C. In other words, the voltages Vg1 to Vg3based on the reference electric current i1 and the threshold voltagesVth1 to Vth3 of the electric current sources 30A to 30C are stored inthe storage units 32A to 32C, respectively.

The electric current sources 30A to 30C supply the comparator circuits21A to 21C with electric currents generated by the voltages based on thereference electric current i1 from the reference electric current sourceI1 stored in the storage units 32A to 32C, respectively. In other words,the electric current sources 30A to 30C supply the comparator circuits21A to 21C with electric currents generated by the voltages Vg1 to Vg3based on the reference electric current i1 from the reference electriccurrent source I1 stored in the storage units 32A to 32C, respectively.

Although the above embodiment illustrates the electric current source 30as an electric current source for the readout circuit 100, which readsout the pixel signal, included in the image sensor 3 and an electriccurrent source for the comparator circuit 21 of the analog/digitalconversion circuit that converts the pixel signal into the digitalsignal, the present invention is not limited to this. The electriccurrent source 30 is also applicable as an electric current source forother source follower circuits in addition to the electronic circuitsincluded in the image sensor 3. The electric current source 30 isfurther applicable to electronic circuits other than the source followercircuits.

According to the above embodiment, the following operational advantagescan be achieved.

(1) The image sensor 3 includes the readout circuit 100 that reads out asignal generated by an electric charge resulting from the photoelectricconversion to the signal line 17, the storage circuit 110 that stores avoltage based on the electric current i1 from the reference electriccurrent source I1, and the electric current supply 30 that supplies thesignal line 17 with an electric current for causing the readout circuit100 to read out a signal and supplies the signal line 17 with anelectric current generated by the voltage stored in the storage circuit110. In the first embodiment, the electric current source 30 suppliesthe signal line 17 with the electric current generated by the voltagestored in the storage circuit 110. The influence of an IR drop can thusbe reduced.

(2) In the first embodiment, after the switch SWO is turned on, theelectric current source 30 generates an electric current based on thevoltage stored in the storage unit 32. The electric current generated bythe electric current source 30 flows to the ground via the wiringresistor (R1 to R3). In the ground wiring, a voltage drop (an IR drop)due to an IR product of the electric current and the resistance occursso that a source voltage of the electric current source 30 increases.Since the storage unit 32 holds a relative voltage between the gate andthe source of the electric current source 30, the voltage Vg increasesas the source voltage of the electric current source 30 increases, sothat fluctuations in the gate-source voltage is reduced. The reductionin fluctuations of the gate-source voltage can lead to a reduction influctuations of the electric current supplied by the electric currentsource 30.

(3) The electric current source 30 includes a transistor having a drainpart connected to the signal line 17 and a gate part connected to thestorage circuit 110 and the drain part. In this way, the storage circuit110 can store a voltage based on the threshold voltage Vth of thetransistor and the reference electric current i1. Additionally, theelectric current source 30 can supply an electric current that is lessinfluenced by fluctuations in the threshold voltage Vth.

(4) The image sensor 3 includes the first readout circuit 100A thatreads out a first signal to the signal line 17A, the first signal beinggenerated by the electric charge resulting from the photoelectricconversion, the second readout circuit 100B that reads out a secondsignal to the signal line 17B, the second signal being generated by theelectric charge resulting from the photoelectric conversion, the firststorage circuit 110A that stores a voltage based on the electric currenti1 from the reference electric current source I1, the second storagecircuit 110B that stores a voltage based on the electric current i1 fromthe reference electric current source I1, the first electric currentsupply 30A that supplies the first signal line 17A with an electriccurrent for causing the first readout circuit 100A to read out a firstsignal and supplies the first signal line 17A with an electric currentgenerated by the voltage stored in the first storage circuit 110A, andthe second electric current supply 30B that supplies the second signalline 17B with an electric current for causing the second readout circuit100B to read out a second signal and supplies the second signal line 17Bwith an electric current generated by the voltage stored in the secondstorage circuit 110B. In the first embodiment, the first current source30A supplies the signal line 17A with the electric current generated bythe voltage stored in the first storage circuit 110A, and the secondcurrent source 30B supplies the signal line 17B with the electriccurrent generated by the voltage stored in the second storage circuit110B. The influence of an IR drop can thus be reduced in a plurality ofsignal lines.

(5) After the voltage based on the electric current i1 from thereference electric current source I1 is stored in the first storagecircuit 110A, the second storage circuit 110B stores the voltage basedon the electric current i1 from the reference electric current sourceI1. In this way, the voltage based on the electric current i1 from thereference electric current source I1 can be sequentially stored in theplurality of storage circuits 110.

(6) The first storage circuit 110A has the first control unit 33A thatcontrols the electric current i1 from the reference electric currentsource I1 when the first readout circuit 100A reads out the first signalto the first signal line 17A to be smaller than that when the firstreadout circuit 100A does not read out the first signal to the firstsignal line 17A, and the second storage circuit 110B has the secondcontrol unit 33B that controls the electric current i1 from thereference electric current source I1 when the second readout circuit100B reads out the second signal to the second signal line 17B to besmaller than that when the second readout circuit 100B does not read outthe second signal to the second signal line 17B. In this way, thereadout circuit 100 can read out the pixel signal to the signal line 17based on the electric current generated by the voltage stored in thestorage circuit 110. Additionally, power consumption can be reduced byadjusting to reduce the electric current i1 from the reference electriccurrent source I1.

(7) The first current source 30A includes the first transistor M10having the first drain part connected to the first signal line 17A andthe first gate part connected to the first storage circuit 110A and thefirst drain part, and the second current source 30B includes the secondtransistor M20 having the second drain part connected to the secondsignal line 17B and the second gate part connected to the second storagecircuit 110B and the second drain part. In this way, the storage circuit110A stores a voltage based on the threshold voltage Vth1 of thetransistor M10 and the reference electric current i1, and the storagecircuit 110B stores a voltage based on the threshold voltage Vth2 of thetransistor M20 and the reference electric current i1. Additionally, thefirst electric current source 30A and the second electric current source30B can supply an electric current that is less influenced byfluctuations in the threshold voltage Vth.

Second Embodiment

Referring to FIG. 7 , an image sensor 3 according to a second embodimentwill be described. It should be noted that parts that are the same orequivalent to those in the first embodiment are denoted by the samereference numerals in the figure, and differences will be mainlydescribed. The image sensor 3 according to the second embodiment furtherincludes an electric current source circuit 50 having a first storageunit 32 that stores a voltage based on the electric current i1 from thereference electric current source I1 and a supply unit 130 that suppliesan electric current generated by the voltage stored in the first storageunit 32, wherein the electric current source 30 generates an electriccurrent to be supplied to the readout circuit 100 based on the electriccurrent from the electric current source circuit 50.

FIG. 7(a) is a circuit diagram illustrating a part of the pixel circuit150, the electric current source circuit 50, and the reference electriccurrent source circuit 31, according to the second embodiment. FIG. 7(b)is a diagram illustrating a connection relationship between a part ofthe pixel circuit 150, the electric current source circuit 50, and thereference electric current source circuit 31 according to the secondembodiment.

An example illustrated in FIG. 7(b) illustrates a pixel circuit 150having eight rows and ten columns. Each pixel circuit 150 includespixels 10 and also includes electric current sources 30 and storagecircuits 110 that are arranged for individual pixels 10. The number ofthe pixel circuits 150 is the same as the number of the pixels 10 of theimage sensor 3. The number of the current source circuits 50 is equal tothe number of columns of the pixel circuits 150 of the image sensor 3.Additionally, the number of the current source circuits 50 may be largerthan the number of columns of the pixel circuits 150 of the image sensor3. For example, the number may be the same as the number of the pixelcircuits 150 in two rows and ten columns. Since the pixel circuit 150has ten columns in FIG. 7(b), ten electric current source circuits 50(current source circuits 50 a to 50 j) are accordingly illustrated inthe figure. As illustrated in FIG. 7(a), each current source circuit 50includes a supply unit 130, a first storage unit 32, and switches SWS1,SWD1, and SWO1. Each current source circuit 50 is supplied with areference electric current i1 from the reference electric current sourceI1 of the reference electric current source circuit 31. Such aconfiguration of the plurality of current source circuits 50 and thereference electric current source circuit 31 are the same as theconfiguration of the electric current source 30 and its peripheralcircuit illustrated in FIG. 3 . It should be noted that the control unit33 may include the switch SWS1, the switch SWD1, the switch SWO1, theswitch SWS2, the switch SWD2, and the switch SWO2. Furthermore, thestorage circuit 110 may include the first storage unit 32, the secondstorage unit 132, the control unit 33, and the supply unit 130.

A connection relationship between the electric current source circuit 50and the pixel circuit 150 will now be described. In FIG. 7(b), theelectric current source circuit 50 a located at the left end isconnected to a plurality of pixel circuits 150 a 1, 150 a 2, 150 a 3,150 a 4, 150 a 5, 150 a 6, 150 a 7, 150 a 8 located at the left end. Inother words, the current source circuit 50 a located at the left end isconnected to each of the plurality of pixel circuits 150 in the left endcolumn. Likewise, an electric current source circuit 50 b next to theelectric current source circuit 50 a in the leftmost column is connectedto a plurality of pixel circuits 150 b 1 to 150 b 8 next to the pixelcircuits 150 a 1 to 150 a 8 in the leftmost column. The same alsoapplies to other columns and the electric current source circuit 50 jlocated at the rightmost is connected to a plurality of pixel circuits150 j 1, 150 j 2, 150 j 3, 150 j 4, 150 j 5, 150 j 6, 150 j 7, 150 j 8in the rightmost column.

The connection between the electric current source circuit 50 and thepixel circuit 150 is made by connecting the switch SWS2 of the pixelcircuit 150 and the switch SWO1 of the electric current source circuit50 as illustrated in FIG. 7(a). This connection allows the capacitor C1of the second storage unit 132 of the pixel circuit 150 to be connectedto the supply unit 130 of the electric current source circuit 50 via theswitches SWD2, SWS2 and the switch SWO1.

For the plurality of current source circuits 50 illustrated in FIG.7(b), a voltage based on the reference electric current i1 from thereference electric current source I1 is sequentially stored in acapacitor C1 of the first storage unit 32 by turning on and off theswitches SWS1 and SWD1, from the electric current source circuit 50 a atthe leftmost to the electric current source circuit 50 j at therightmost, for example. The on and off control of these switches SWS1and SWD1 is performed in the same manner as the on and off control ofthe switches SWS1 and SWD1, the on and off control of the switches SWS2and SWD2, and the on and off control of the switches SWS3 and SWD3, asillustrated in FIG. 4 .

Once the storage of the voltages based on the reference electric currenti1 has been completed in all the electric current source circuits 50 ato 50 j, the switches SWO1 of all the electric current source circuits50 a to 50 j are simultaneously turned on in the same manner as theswitches SWO1 to SWO3 illustrated in FIG. 4 . With the switches SWO1,the supply units 130 for all the electric current source circuits 50 ato 50 j are ready to supply the pixel circuits 150 a to 150 j withelectric currents based on the reference electric current i1. The supplyunit 130 supplies the electric current generated by the voltage storedin the first storage unit 32. The electric current from the electriccurrent source circuit 50 is the same or substantially the same as thereference electric current i1 from the reference electric current sourceI1. In this manner, each of the electric current source circuits 50 a to50 j can generate an electric current based on the reference electriccurrent i1 of the reference electric current source I1 and supply it tothe pixel circuits 150 a to 150 j.

Then, all the switches SWS2, SWD2 of the plurality of pixel circuits 150al to 150 j 1 in the lowermost row are simultaneously turned on. Oncethe switches are turned on in such a manner, a voltage is stored in thecapacitor C1 of the second storage unit 132 by an electric current fromthe supply unit 130 of each current source circuit 50 a to 50 j in thepixel circuits 150 al to 150 j 1 in the lowermost row. In this manner,voltages based on the electric currents from the plurality of electriccurrent source circuits 50 a to 50 j are simultaneously stored in thesecond storage units 132 of the plurality of pixel circuits 150 a 1 to150 j 1 in the lowermost row.

Thereafter, the switches SWS2 and SWD2 are simultaneously turned on inall the plurality of pixel circuits 150 a 2 to 150 j 2 in a second rowfrom the bottom, so that voltages are simultaneously stored in thesecond storage units 132 by electric currents of individual currentsource circuits 50 a to 50 j. Likewise, voltages based on the electriccurrents from the plurality of electric current source circuits 50 a to50 j are simultaneously stored in the second storage units 132 of theplurality of pixel circuits 150 a 8 to 150 j 8 in the uppermost row.Once the switch SWO2 is turned on after the voltage is stored in thesecond storage unit 132, the electric current source 30 supplies thesignal line 17 with an electric current based on the voltage stored inthe second storage unit 132.

It should be noted that the storage circuit 110 may store the voltagebased on the reference electric current i1 from the reference electriccurrent source I1 in the first storage unit 32 while the readout circuit100 reads out the signal to the signal line 17. For example, in thestorage circuit 110, while the switches SWS2 and SWD2 are turned off andthe switch SWO2 is turned on to read out the signal to the signal line17 by the readout circuit 110, the switches SWS1 and SWD1 are turned onto supply the reference electric current i1 from the reference electriccurrent source I1 to the first storage unit 32. In response to thesupply of the reference electric current i1, the first storage unit 32stores the voltage based on the reference electric current i1.

Although the above embodiment illustrates the electric current source 30as an electric current source for the readout circuit 100, which readsout the pixel signal, included in the image sensor 3, the presentinvention is not limited to this. The electric current source 30 is alsoapplicable as an electric current source for the comparator circuit 21of the analog/digital conversion circuit that converts the pixel signalinto the digital signal and an electric current source for other sourcefollower circuits in addition to the electronic circuits included in theimage sensor 3. The electric current source 30 is further applicable toelectronic circuits other than the source follower circuits.

According to the above embodiment, the following operational advantagescan be achieved in addition to the same operational advantages as in thefirst embodiment.

(8) The storage circuit 110 includes the first storage unit 32 thatstores a voltage based on the electric current i1 from the referenceelectric current source I1, a supply unit 130 that supplies the electriccurrent generated by the voltage stored in the first storage unit 32,and a second storage unit 132 that stores a voltage based on theelectric current supplied from the supply unit 130, wherein the electriccurrent source 30 supplies the signal line 17 with the electric currentgenerated by the voltage stored in the second storage unit 132. In thisway, it is possible to generate an electric current in the supply unit130 based on the reference electric current i1 of the reference electriccurrent source I1 and cause the electric current source 30 to generatean electric current based on the electric current from the supply unit130.

(9) The storage circuit 110 includes the control unit 33 that isprovided between the supply unit 130 and the second storage unit 132 andcontrols the electric current from the reference electric current sourceI1 to the first storage unit 32 when the read out circuit 100 reads outa signal to the signal line 17 to be smaller than that when the read outcircuit 100 does not read out a signal to the signal line 17. In thisway, the readout circuit 100 can read the pixel signal to the signalline 17 based on the electric current generated by the voltage stored inthe storage circuit 110. Additionally, power consumption can be reducedby adjusting to reduce the electric current i1 from the referenceelectric current source I1.

Third Embodiment

FIG. 8 is a circuit diagram illustrating a configuration of an electriccurrent source 30 and a storage circuit 110 according to a thirdembodiment. It should be noted that parts that are the same orequivalent to those in the first embodiment are denoted by the samereference numerals in the figure, and differences will be mainlydescribed. The first embodiment illustrated in FIG. 3 describes anexample in which the reference electric current source I1 of thereference electric current source circuit 31 sequentially supplies thestorage circuits 110A to 110C with the reference electric current i1 tostore voltages based on the threshold voltages of the electric currentsources 30A to 30C and the reference electric current i1. Contrastingly,in the third embodiment, the reference electric current source I1 of thereference electric current source circuit 31 supplies the referencevoltage generator 60 with the reference electric current i1 to generatea reference voltage Vb and then store the reference voltage Vb in thestorage circuits 110A to 110C in common.

In the third embodiment, the reference electric current source circuit31 further includes the reference voltage generator 60 in addition tothe reference electric current source I1. The reference voltagegenerator 60 includes a transistor M70, for example, and generates thereference voltage Vb based on the reference electric current i1 suppliedby the reference electric current source I1 and the threshold voltage ofthe reference voltage generator 60. Control units 33A to 33C accordingto the third embodiment include switches SWS1 to SWS3 and switches SWO1to SWO3, respectively.

The storage unit 32A and the switch SWS1, the storage unit 32B and theswitch SWS2, and the storage unit 32C and the switch SWS3 are connectedin parallel to the reference voltage generator 60. When the switchesSWS1 to SWS3 are simultaneously turned on, the reference voltage Vb isstored in the storage units 32A to 32C.

In this way, since the reference voltage Vb generated by the referencevoltage generator 60 is stored in the storage units 32A to 32C in commonin the third embodiment, the switches SWS1 to SWS3 can be simultaneouslyturned on to store the reference voltage Vb.

When the switches SWS1 to SWS3 are turned off, capacitors C1 to C3respectively constituting the storage units 32A to 32C hold thereference voltage Vb. When the switches SWO1 to SWO3 are turned on, theelectric current sources 30A to 30C generate electric currents based onthe reference voltage Vb stored in the storage units 32A to 32C,respectively, to supply the electric currents to the readout circuits100 as a supply destinations.

Although the above embodiment illustrates the electric current source 30as an electric current source for the readout circuit 100, which readsout the pixel signal, included in the image sensor 3, the presentinvention is not limited to this. The electric current source 30 is alsoapplicable as an electric current source for the comparator circuit 21of the analog/digital conversion circuit that converts the pixel signalinto the digital signal and an electric current source for other sourcefollower circuits in addition to the electronic circuits included in theimage sensor 3. The electric current source 30 is further applicable toelectronic circuits other than the source follower circuits.

According to the above embodiment, the following operational advantagescan be achieved in addition to the same operational advantages as in thefirst embodiment.

(10) In the third embodiment, the image sensor 3 further includes thereference voltage generator 60 that generates the reference voltage Vbbased on the reference electric current i1. In this way, it is possibleto simultaneously store the reference voltage Vb based on the referenceelectric current i1 in the plurality of storage circuits 110A to 110C.

The following variations are also contemplated within the scope of thepresent invention, and one or more variations may be combined with theabove embodiments.

First Variation

FIG. 9(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to afirst variation. An image sensor 3 according to the first variationfurther includes a precharge unit 70 and a switch SWSr. Electric currentsource circuits 50 (50A to 50C) according to the first variation arecomposed of electric current sources and storage units that are the sameas the electric current sources 30A to 30C and the storage units 32A to32C, respectively, illustrated in FIG. 3 .

The precharge unit 70 includes a diode-connected transistor M80. Theelectric current source circuit 50A, the electric current source circuit50B, and the electric current source circuit 50C are connected inparallel to the precharge unit 70 and the switch SWSr. Before each ofthe electric current source circuits 50A to 50C is supplied with thereference electric current i1, the precharge unit 70 applies a prechargevoltage to a node 80 illustrated in FIG. 9(a) via the switch SWSr.

FIG. 9(b) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the first variation. Although not illustrated, it is assumed that theswitches SWD1 and SWD3 are turned on at the same time as the switchesSWS1 and SWS3, respectively, and are turned off prior to the switchesSWS1 and SWS3.

At a time t1, a control signal of the switch SWSr is turned to highlevel so that the switch SWSr is turned on, and the reference electriccurrent i1 is supplied to the precharge unit 70. The gate-source voltageVgs of the precharge unit 70 becomes a predetermined value depending onthe reference electric current i1 and the threshold voltage of theprecharge unit 70. At the node 80, the gate-source voltage Vgs of theprecharge unit 70 is set as the precharge voltage.

At a time t2, the control signal of the switch SWSr is turned to lowlevel and the control signal of the switch SWS1 is turned to high level.Turning on the switch SWS1 allows the reference electric current i1 tobe supplied to the electric current source circuit 50A via the node 80to store a voltage in the storage unit 32A of the electric currentsource circuit 50A.

The time until the voltage based on the reference electric current i1 isreached can be shortened by setting the precharge voltage at the node 80before the supply of the reference electric current i1, since storingthe voltage begins from the precharge voltage level in the storage unit32A of the electric current source circuit 50A.

In a period from a time t3 to a time t5, the voltage based on thereference electric current i1 is sequentially stored in the electriccurrent source circuits 50B to 50C. At a time t6, the switches SWO1 toSWO3 are turned on so that each current source circuit 50 supplies thereadout circuit 100 with an electric current based on the referenceelectric current i1.

FIG. 9(c) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the first variation. In FIG. 9(b) described above, the switches SWO1to SWO3 are simultaneously turned on after the switches SWS1 to SWS3 aresequentially turned off. Contrastingly, in an example illustrated inFIG. 9(c), the switch SWO1 is turned on immediately after the switchSWS1 is turned on. Likewise, the switches SWO2, SWO3 are respectivelyturned on immediately after the switches SWS2 and SWS3 are turned on.

Although the first and second embodiments describe an example in whichthe switches SWO1 to SWO3 are simultaneously turned on. However, theswitches SWO1 to SWO3 may be sequentially turned on in the same manneras in the example illustrated in FIG. 9(c).

Second Variation

FIG. 10(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to asecond variation. An image sensor 3 according to the second variationfurther includes a switch SWX as compared with the first variation.

An EN signal inputted to the reference electric current source I1 is asignal for controlling the generation of the reference electric currenti1 by the reference electric current source I1. The reference electriccurrent source I1 generates the reference electric current i1 if the ENsignal is at its high level and does not generate the reference electriccurrent i1 if the EN signal is at its low level.

FIG. 10(b) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the second variation. Although not illustrated, it is assumed thatthe switches SWD1 and SWD3 are turned on at the same time as theswitches SWS1 and SWS3, respectively, and are turned off prior to theswitches SWS1 and SWS3.

At a time t1, the EN signal and the control signal of the switch SWX areturned to high level and the control signal of the switch SWSr is turnedto high level. With the EN signal at its high level, the referenceelectric current source I1 is ready to output the reference electriccurrent i1. The switches SWX and SWSr are turned on to connect theprecharge unit 70 and the reference electric current source circuit 31.The precharge unit 70 generates a precharge voltage based on thereference electric current i1 and applies the precharge voltage to thenode 80.

In a period from a time t2 to a time t5, the voltage based on thereference electric current i1 is sequentially stored in the electriccurrent source circuits 50A to 50C in the same manner as in the firstvariation. Furthermore, at a time t5, the EN signal and the controlsignal of the switch SWX are turned to low level. With the EN signal atits low level, the reference electric current source I1 does notgenerate the reference electric current i1. Power consumption can bereduced by stopping the generation of the reference electric current i1.Turning off the switch SWX to disconnect the reference electric currentsource I1 and the node 80 prevents the voltage of the node 80 fromdropping through the reference electric current source I1. At a time t6,the switches SWO1 to SWO3 are turned on so that the current sourcecircuits 50A to 50C supply the readout circuits 100 with electriccurrents based on the reference electric current i1.

FIG. 10(c) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the second variation. In the example illustrated in FIG. 10(c), theswitch SWO1 is turned on immediately after the switch SWS1 is turned on.Likewise, the switches SWO2, SWO3 are respectively turned on immediatelyafter the switches SWS2 and SWS3 are turned on.

Third Variation

FIG. 11(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to athird variation. An image sensor 3 according to the third variationincludes reference electric current source circuits 31A to 31C. Thereference electric current source circuits 31A to 31C include referenceelectric current sources I1 to I3, respectively. The reference electriccurrent sources I1 to I3 generate reference electric currents i1 to i3having different current values, respectively. Electric current sourcecircuits 50 according to the third variation are composed of electriccurrent sources and storage units that are the same as the electriccurrent sources 30 and the storage units 32, respectively, illustratedin FIG. 3 .

In this variation, the reference electric current to be supplied to eachelectric current source circuit 50 can be switched. FIG. 11(a)illustrates only three electric current source circuits 50 (electriccurrent source circuits 50A to 50C). Variations in the electric currentgenerated by each current source circuit 50 can be reduced by switchingthe reference electric current, which is supplied in advance, inaccordance with characteristics of the control signal inputted to theswitch SWD constituting each electric current source circuit 50.

FIG. 11(b) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the third variation. Although not illustrated, it is assumed that theswitch SWD is turned on at the same time as the switch SWS and is turnedoff prior to the switch SWS.

At a time t1, the ENa signal and the control signal of the switch SWXaare turned to high level and the control signal of the switch SWSr isturned to high level. With the ENa signal at its high level, thereference electric current source I1 is ready to output the referenceelectric current i1. The switches SWX and SWSr are turned on to connectthe precharge unit 70 and the reference electric current source I1. Theprecharge unit 70 generates a precharge voltage based on the referenceelectric current i1 and applies the precharge voltage to the node 80.

At a time t2, the control signal of the switch SWSr is turned to lowlevel and the control signal of the switch SWS1 is turned to high level.Turning on the switch SWS1 allows the reference electric current i1 tobe supplied to the electric current source circuit 50A via the node 80to store a voltage based on the reference electric current i1. In aperiod from a time t3 to a time t5, the voltage based on the referenceelectric current i1 is sequentially stored in the electric currentsource circuits 50B to 50C.

At a time t5, the ENa signal and the control signal of the switch SWXaare turned to low level, the ENb signal and the control signal of theswitch SWXb are turned to high level, and the control signal of theswitch SWSr is turned to high level. With the ENb signal at its highlevel, the reference electric current source I2 is ready to output areference electric current i2. The switches SWXb and SWSr are turned onto connect the precharge unit 70 and the reference electric currentsource I2. The precharge unit 70 generates a precharge voltage based onthe reference electric current i2 and applies the precharge voltage tothe node 80.

In a period from a time t6 to a time t9, a voltage based on thereference electric current i2 is sequentially stored in electric currentsource circuits 50D to 50F (not shown) connected to SWS4 to SWS6,respectively, in the same manner as in the period from the time t2 tothe time t5.

Furthermore, at a time t9, the ENb signal and the control signal of theswitch SWXb are turned to low level, the ENc signal and the controlsignal of the switch SWXc are turned to high level, and the controlsignal of the switch SWSr is turned to high level. With the ENc signalat its high level, the reference electric current source I3 is ready tooutput the reference electric current 3. The switches SWXc and SWSr areturned on to connect the precharge unit 70 and the reference electriccurrent source I3. The precharge unit 70 generates a precharge voltagebased on the reference electric current i3 and applies the prechargevoltage to the node 80.

In a period from a time t10 to a time t13, a voltage based on thereference electric current i3 is sequentially stored in electric currentsource circuits 50G to 50I (not shown) connected to the SWS7 to SWS9,respectively, in the same manner as in the period from the time t2 tothe time t5.

At a time t13, the control signals of the switches SWO1 to SWO9 aresimultaneously are turned to high level. The switches SWO1 to SWO9 areturned on so that the electric current source circuits 50A to 50I supplyelectric currents based on the voltages stored therein to theirconnection destinations.

Fourth Variation

FIG. 12(a) is a diagram illustrating a configuration of an electriccurrent source circuit 50 and its peripheral circuits according to afourth variation. An image sensor 3 according to the fourth variationfurther includes a switch SWSb. A control signal of the switch SWSb isturned to high level so that the switch SWSb is turned on. This supplieseach electric current source circuit 50A to 50C with a ground level.Electric current source circuits 50 according to the fourth variationare composed of electric current sources and storage units that are thesame as the electric current sources and the storage units illustratedin FIG. 3 .

If pixel signals are read out from only some pixels 10 among all thepixels 10 of the image sensor 3, other pixels 10 are inactivated, thatis, the electric current source 30 is disabled to generate electriccurrents. This can reduce power consumption. The electric current source30 provides a power supply VDD level voltage if the electric currentsource 30 is composed of a PMOS transistor, and provides a ground levelvoltage if the electric current source 30 is composed of an NMOStransistor, so that the electric current source 30 is disabled togenerate electric current. Here, the pixels 10 that allow the electriccurrent source 30 to generate electric current are referred to as activepixels, and the pixels 10 that do not allow the electric current source30 to generate electric current are referred to as inactive pixels.

FIG. 12(b) is a diagram illustrating a configuration example of a partof the pixel 10 according to the fourth variation. In FIG. 12(b),hatched pixels 10 indicate inactive pixels and white pixels 10 indicateactive pixels.

For example, from a first image frame, active pixels are determined asregions of interest. In a second image frame, voltages are stored instorage units 32 of inactive pixels so that the pixels are inactive,while voltages of storage units 32 of the active pixels are reset, thatis, refreshing of the voltages is performed. Since refreshing of thevoltages of the storage units 32 of the inactive pixels is unnecessary,the time required for refreshing of the voltages can be shortened. Forexample, for the line B-B′ illustrated in FIG. 12(b), refreshing ofvoltages of the storage units 32 at third and subsequent frames can beeliminated.

FIG. 12(b) is a timing chart illustrating an exemplary operation of theelectric current source circuit 50 and its peripheral circuits accordingto the fourth variation. Although not illustrated, it is assumed thatthe switch SWD is turned on at the same time as the switch SWS and isturned off prior to the switch SWS.

In the example illustrated in FIG. 12(c), a voltage based on thereference electric current i1 is sequentially stored from the electriccurrent source circuit (50A) connected to the leftmost pixel 10 to theelectric current source circuit (50J) connected to the rightmost pixel10 in a line A-A′ illustrated in FIG. 12(b). At a time t1, the controlsignal of the switch SWSb is turned to high level and the control signalof the switch SWS1 is turned to high level. The switch SWSb and theswitch SWS1 are turned on, so that a ground level (0 V) is stored in thestorage unit 32A of the electric current source circuit 50A.

At a time t2, the EN signal and the control signal of the switch SWX areturned to high level, the control signal of the switch SWSb is turned tolow level, and the control signal of the switch SWS2 is turned to highlevel. With the EN signal at its high level, the reference electriccurrent source I1 is ready to output the reference electric current i1.Turning on the switches SWX and SWS2 allows the reference electriccurrent i1 to be supplied to the electric current source circuit 50B tostore a voltage based on the reference electric current i1. Likewise, ina period from a time t3 to a time t5, electric current source circuits50C to 50D (not shown) are supplied with the reference electric currenti1 to store a voltage based on the reference electric current i1.

Furthermore, at a time t5, the EN signal and the control signal of theswitch SWX are turned to low level, the control signal of the switchSWSb is turned to high level, and the control signal of the switch SWS5is turned to high level. With the EN signal at its low level, thereference electric current source I1 stops the generation of thereference electric current i1. The switch SWSb and the switch SWS5 areturned on, so that a ground level is stored in the electric currentsource circuit 50E (not shown). Likewise, in a period from a time t6 toa time t7, the ground level is stored in the electric current sourcecircuit 50F (not shown).

At a time t7, the EN signal and the control signal of the switch SWX areturned to high level, the control signal of the switch SWSb is turned tolow level, and the control signal of the switch SWS7 is turned to highlevel. Turning on the switches SWX and SWS7 allows the referenceelectric current i1 to be supplied to the electric current sourcecircuit 50G (not shown) to store a voltage based on the referenceelectric current i1. Likewise, in a period from a time t8 to a time t10,the reference electric current i1 is supplied in electric current sourcecircuits 50H to 50I (not shown) to set a voltage based on the referenceelectric current i1.

Furthermore, at a time t10, the EN signal and the control signal of theswitch SWX are turned to low level, the control signal of the switchSWSb is turned to high level, and the control signal of the switch SWS10is turned to high level. The switch SWSb and the switch SWS10 are turnedon, so that a ground level is stored in the electric current sourcecircuit 50J (not shown).

At a time t12, the switches SWO1 to SWO10 are turned on so that theelectric current source circuits 50A to 50J generate electric currentsbased on the voltages stored in respective storage units 32 to besupplied to their connection destinations.

Fifth Variation

The above embodiment describes an example in which each electric currentsource 30 is composed of one transistor. However, as in the exampleillustrated in FIGS. 13(a) to 13(d), the electric current source 30 mayinclude a cascode connection of a plurality of transistors M10 and M20.The electric current source 30 as a cascode configuration of thetransistors can increase an output impedance of the electric currentsource 30 and reduce fluctuations in the electric current supplied bythe electric current source 30.

FIGS. 13(a) to 13(d) are diagrams illustrating a configuration of anelectric current source 30 and its peripheral circuits according to afifth variation. FIGS. 14(a) to 14(c) are timing charts illustrating anexemplary operation of the electric current source 30 and its peripheralcircuits according to the fifth variation. In the example illustrated inFIG. 13(a), the electric current source 30 and its peripheral circuitsinclude reference electric current sources I1 and I2, transistors M10,M20, and M70, switches SWS, SWD, SWO, and SWC, and capacitors C1 and C2.The transistor M70 is supplied with a reference electric current i2 fromthe reference electric current source I2 to generate a voltage V2 basedon the reference electric current i2 and a threshold voltage of thetransistor M70.

As illustrated in FIG. 14(a), at a time t1, the switch SWC is turned onso that the capacitor C2 and a gate terminal of the transistor M20 aresupplied with a voltage V2. Furthermore, the switches SWS and SWD areturned on so that a voltage V1 based on the reference electric currenti1 from the reference electric current source I1 and the thresholdvoltage of the transistor M10 is stored in the capacitor C1. At a timet2, the switch SWD is turned off so that the voltage V1 is held in thecapacitor C1. At a time t3, the switches SWC and SWS are turned off sothat the voltage V2 is held in the capacitor C2. At a time t4, theswitch SWO is turned on, and the electric current source 30 supplies anelectric current to its connection destination.

In an example illustrated in FIG. 13(b), a connection destination of thecapacitor C2 is different from that in FIG. 13(a). As illustrated inFIG. 14(b), at a time t1, the voltage V2 is set at the capacitor C2 andthe gate terminal of the transistor M20 in the same manner as in FIG.13(a). A voltage V2 is stored in the capacitor C2 with reference to apotential of the capacitor C1 and a potential of a gate terminal of thetransistor M10. Furthermore, a voltage V1 is stored in the capacitor C1in the same manner as in FIG. 13(a).

At a time t2, the switch SWD is turned off so that the voltage V1 isheld in the capacitor C1. The switch SWS is turned off at a time t3 andthe switch SWC is turned off at a time t4 so that the voltage V2 is heldin the capacitor C2. At a time t5, the electric current source 30supplies an electric current to its connection destination.

In the example illustrated in FIGS. 13(c) to 13(d), the electric currentsource 30 and its peripheral circuits include the reference electriccurrent source I1, transistors M10 and M20, switches SWS, SWD, SWO, andSWC, and capacitors C1 and C2. The voltage V1 based on the referenceelectric current i1 and the threshold voltage of the transistor M10 isstored in the capacitor C1 and the voltage V2 based on the referenceelectric current i1 and the threshold voltage of the transistor M20 isstored in the capacitor C2. In the example illustrated in FIG. 13(c),the voltage V2 is stored in the capacitor C2 with reference to a groundpotential. In the example illustrated in FIG. 13(d), a voltage V2 isstored in the capacitor C2 with reference to a potential of thecapacitor C1 and a potential of a gate terminal of the transistor M10.On and off control is the same for the switches illustrated in FIGS.13(c) and 13(d), and a control signal illustrated in FIG. 14(c) isinputted.

As illustrated in FIG. 14(c), at a time t1, a voltage V2 is stored inthe capacitor C2 and a voltage V1 is stored in the capacitor C1. At atime t2, the switch SWD is turned off so that the voltage V1 is held inthe capacitor C1. At a time t3, the switch SWC is turned off so that thevoltage V2 is held in the capacitor C2. The switch SWS is turned off ata time t4 and the switch SWO is turned off at a time t5 so that theelectric current source 30 supplies an electric current to itsconnection destination.

Sixth Variation

An image sensor 3 may be made of a single semiconductor substrate or aplurality of semiconductor substrates stacked each other. For example,the image sensor 3 is provided with a first semiconductor substrateprovided with a readout circuit 100 and a second semiconductor substrateprovided with a first storage unit 32 and a supply unit 130.

Although the above embodiments and variations illustrate the electriccurrent source 30 as an electric current source for the readout circuit100, which reads out the pixel signal, included in the image sensor 3and an electric current source for the comparator circuit 21 of theanalog/digital conversion circuit that converts the pixel signal intothe digital signal, the present invention is not limited to this. Theelectric current source 30 is also applicable as an electric currentsource for other source follower circuits in addition to the electroniccircuits included in the image sensor 3. The electric current source 30is further applicable to electronic circuits other than the sourcefollower circuits.

While various embodiments and variations have been described above, thepresent invention is not limited to these. Other aspects contemplatedwithin the technical idea of the present invention are also includedwithin the scope of the present invention.

The embodiments and variations described above also include thefollowing image sensors and electric current source circuits.

(1) An image sensor, comprising: a readout circuit that reads out asignal to a signal line, the signal being generated by an electriccharge resulting from a photoelectric conversion; a holding circuit thatholds a voltage based on an electric current from a power supplycircuit; and an electric current source including a transistor having adrain part connected to the signal line and a gate part connected to theholding circuit and the drain part, the electric current sourcesupplying the signal line with an electric current generated by thevoltage held in the holding circuit.

(2) In the image sensor as recited in (1), the holding circuit includesa first holding unit that holds the voltage based on the electriccurrent from the power supply circuit, a supply unit that supplies anelectric current generated by the voltage held in the first holdingunit, and a second holding unit that holds a voltage based on theelectric current supplied from the supply unit; and the electric currentsource supplies the signal line with an electric current generated bythe voltage held in the second holding unit.

(3) In the image sensor as recited in (2), the holding circuit has acontrol unit that is provided between the supply unit and the secondholding unit and controls an electric current flowing from the powersupply circuit to the first holding unit when the readout circuit readsout the signal to the signal line to be smaller than that when thereadout circuit does not read out the signal to the signal line.

(4) In the image sensor as recited in (2) or (3), the holding circuitholds a voltage based on the electric current from the power supplycircuit in the first holding unit when the readout circuit reads out thesignal to the signal line.

(5) In the image sensor as recited in (2) or (3), the first holding unitand the supply unit are provided on a second semiconductor substratedifferent from the first semiconductor substrate provided with thereadout circuit.

(6) In the image sensor as recited in (5), the first semiconductorsubstrate is stacked by the second semiconductor substrate.

(7) An image-capturing apparatus comprising the image sensor as recitedin (1) to (6).

(8) An image sensor, comprising: a first readout circuit that reads outa first signal to a first signal line, the first signal being generatedby an electric charge resulting from a photoelectric conversion; asecond readout circuit that reads out a second signal to a second signalline, the second signal being generated by an electric charge resultingfrom a photoelectric conversion; a first holding circuit that holds avoltage based on an electric current from a power supply circuit; asecond holding circuit that holds a voltage based on the electriccurrent from the power supply circuit; a first electric current sourceincluding a first transistor having a first drain part connected to thefirst signal line and a first gate part connected to the first holdingcircuit and the first drain part, the first electric current sourcesupplying the first signal line with an electric current generated bythe voltage held in the first holding circuit; and a second electriccurrent source including a second transistor having a second drain partconnected to the second signal line and a second gate part connected tothe second holding circuit and the second drain part, the secondelectric current source supplying the second signal line with anelectric current generated by the voltage held in the second holdingcircuit.

(9) In the image sensor as recited in (8), the second holding circuitholds a voltage based on the electric current from the power supplycircuit after the voltage based on the electric current from the powersupply circuit is held in the first holding circuit.

(10) In the image sensor as recited in (8) or (9), the first holdingcircuit has a first control unit that controls an electric current fromthe power supply circuit when the first readout circuit reads out thefirst signal to the first signal line to be smaller than that when thefirst readout circuit does not read out the first signal to the firstsignal line; and the second holding circuit has a second control unitthat controls an electric current from the power supply circuit when thesecond readout circuit reads out the second signal to the second signalline to be smaller than that when the second readout circuit does notread out the second signal to the second signal line.

(11) In the image sensor as recited in (8), the first holding circuitincludes a first holding unit that holds a voltage based on the electriccurrent from the power supply circuit, a first supply unit that outputsan electric current generated by the voltage stored in the first holdingunit, and a second holding unit that holds a voltage based on theelectric current supplied from the first supply unit; the second holdingcircuit includes a third holding unit that holds a voltage based on theelectric current from the power supply circuit, a second supply unitthat outputs an electric current generated by the voltage stored in thethird holding unit, and a fourth holding unit that holds a voltage basedon the electric current supplied from the second supply unit; the firstcurrent source supplies the first signal line with an electric currentgenerated by the voltage held in the second holding unit; and the secondcurrent source supplies the second signal line with an electric currentgenerated by the voltage held in the fourth holding unit.

(12) In the image sensor as recited in (11), the first holding circuithas a first control unit that is provided between the first supply unitand the second holding unit and controls an electric current flowingfrom the power supply circuit to the second holding unit when the firstreadout circuit reads out the first signal to the first signal line tobe smaller than that when the first readout circuit does not read outthe first signal to the first signal line; and the second holdingcircuit has a second control unit that is provided between the secondsupply unit and the fourth holding unit and controls an electric currentflowing from the power supply circuit to the fourth holding unit whenthe second readout circuit reads out the second signal to the secondsignal line to be smaller than that when the second readout circuit doesnot read out the second signal to the second signal line.

(13) In the image sensor as recited in (11) or (12), the first holdingcircuit holds a voltage based on the electric current from the powersupply circuit in the first holding unit when the first readout circuitreads out the first signal to the first signal line; and the secondholding circuit holds a voltage based on the electric current from thepower supply circuit in the fourth holding unit when the second readoutcircuit reads out the second signal to the second signal line.

(14) In the image sensor as recited in (11) to (13), the third holdingunit holds a voltage based on the electric current from the power supplycircuit after the voltage based on the electric current from the powersupply circuit is held in the first holding unit.

(15) In the image sensor as recited in (11) to (14), the first holdingunit, the second holding unit, the first supply unit, and the secondsupply unit are provided on a second semiconductor substrate differentfrom the first semiconductor substrate provided with the first readoutcircuit and the second readout circuit.

(16) In the image sensor as recited in (15), the first semiconductorsubstrate is stacked by the second semiconductor substrate.

(17) An image-capturing apparatus comprising the image sensor as recitedin (8) to (16).

(18) An electronic device, comprising: an electronic circuit having aplurality of electronic elements; a holding circuit that holds a voltagebased on an electric current from a power supply circuit; and anelectric current source including a transistor having a drain partconnected to the electronic circuit and a gate part connected to theholding circuit and the drain part, the electric current sourcesupplying the electronic circuit with an electric current generated bythe voltage held in the holding circuit.

(19) In the electronic device as recited in (18), the electronic devicecomprises a first control unit and a second control unit that areprovided between the power supply circuit and the holding circuit andcontrol an electrical connection between the power supply circuit andthe holding circuit, and the holding circuit and the drain part areconnected to each other via the first control unit; and the power supplycircuit and the first control unit are connected to each other via thesecond control unit.

(20) In the electronic device as recited in (18) or (19), the holdingcircuit includes a first holding unit that holds the voltage based onthe electric current from the power supply circuit, a supply unit thatsupplies an electric current generated by the voltage held in the firstholding unit, and a second holding unit that holds a voltage based onthe electric current supplied from the supply unit; and the electriccurrent source supplies the electronic circuit with the electric currentgenerated by the voltage held in the second holding unit.

(21) In the electronic device as recited in (20), the holding circuitholds a voltage based on the electric current from the power supplycircuit in the first holding unit during an operation of the electroniccircuit.

(22) In the electronic device as recited in (18) to (21), the powersupply circuit includes a plurality of electric current source circuitsthat supply electric currents having different electric current values;and the holding circuit holds a voltage based on an electric currentfrom a selected electric current source circuit among the plurality ofelectric current source circuits.

(23) In the electronic device as recited in (18) to (22), the electronicdevice comprises a third control unit that disables the electric currentsource to supply an electric current to the electronic circuit.

(24) In the electronic device as recited in (23), the third control unitis connected to a ground.

(25) In the electronic device as recited in (18) to (24), the electriccurrent source includes a cascode connection of a plurality oftransistors including the transistor.

(26) In the electronic device as recited in (20) or (21), the firstholding unit and the supply unit are provided on a second semiconductorsubstrate different from the first semiconductor substrate provided withthe electronic circuit.

(27) In the electronic device as recited in (26), the firstsemiconductor substrate is stacked by the second semiconductorsubstrate.

(28) In the electronic device as recited in (18) to (27), the electroniccircuit is a readout circuit that reads out a signal generated by anelectric charge resulting from a photoelectric conversion.

(29) In the electronic device as recited in (18) to (27), the electroniccircuit is a comparator constituting a convertor that converts an analogsignal into a digital signal.

Additionally, the embodiments and variations described above alsoinclude the following image sensors and image-capturing apparatuses.

(1) An image sensor, comprising: a readout circuit that reads out asignal to a signal line, the signal being generated by an electriccharge resulting from a photoelectric conversion; a storage circuit thatstores a voltage based on an electric current from a reference electriccurrent source; an electric current source that supplies the signal linewith an electric current for causing the readout circuit 100 to read outthe signal, and supplies the signal line with an electric currentgenerated by the voltage stored in the storage circuit.

(2) In the image sensor as recited in (1), the storage circuit includes:a first storage unit that stores a voltage based on an electric currentfrom the reference electric current source; a supply unit that suppliesan electric current generated by the voltage stored in the first storageunit; and a second storage unit that storages a voltage based on anelectric current supplied from the supply unit, wherein the electriccurrent source supplies the signal line with an electric currentgenerated by the voltage stored in the second storage unit.

(3) In the image sensor as recited in (2), the storage circuit has anadjustment unit that is provided between the supply unit and the secondstorage unit and adjusts an electric current flowing from the referenceelectric current source to the first storage unit when the readoutcircuit reads out the signal to the signal line to be smaller than thatwhen the readout circuit does not read out the signal to the signalline.

(4) In the image sensor as recited in (3), the storage circuit stores avoltage based on the electric current from the reference electriccurrent source in the first storage unit when the readout circuit readsout the signal to the signal line.

(5) In the image sensor as recited in (1) to (4), the electric currentsource has a transistor including a drain part connected to the signalline and a gate part connected to the storage circuit and the drainpart.

(6) In the image sensor as recited in (2) to (4), the first storage unitand the supply unit are provided on a second semiconductor substratedifferent from the first semiconductor substrate provided with thereadout circuit.

(7) In the image sensor as recited in (6), the first semiconductorsubstrate is stacked by the second semiconductor substrate.

(8) An image-capturing apparatus comprising the image sensor as recitedin (1) to (7).

(9) An image sensor, comprising: a first readout circuit that reads outa first signal to a first signal line, the first signal being generatedby an electric charge resulting from a photoelectric conversion; asecond readout circuit that reads out a second signal to a second signalline, the second signal being generated by an electric charge resultingfrom a photoelectric conversion; a first storage circuit that stores avoltage based on an electric current from a reference electric currentsource; a second storage circuit that stores a voltage based on anelectric current from the reference electric current source; a firstelectric current source that supplies the first signal line with anelectric current for causing the first readout circuit 100 to read outthe first signal, and supplies the first signal line with an electriccurrent generated by the voltage stored in the first storage circuit,and a second electric current source that supplies the second signalline with an electric current for causing the second readout circuit 100to read out the second signal, and supplies the second signal line withan electric current generated by the voltage stored in the secondstorage circuit.

(10) In the image sensor as recited in (9), the second storage circuitstores a voltage based on the electric current from the referenceelectric current source after the voltage based on the electric currentfrom the reference electric current source is stored in the firststorage circuit.

(11) In the image sensor as recited in (9) or (10), the first storagecircuit has a first adjustment unit that adjusts an electric currentfrom the reference electric current source when the first readoutcircuit reads out the first signal to the first signal line to besmaller than that when the first readout circuit does not read out thefirst signal to the first signal line, and the second storage circuithas a second adjustment unit that adjusts an electric current from thereference electric current source when the second readout circuit readsout the second signal to the second signal line to be smaller than thatwhen the first readout circuit does not read out the second signal tothe second signal line

(12) In the image sensor as recited in (9), the first storage circuitincludes a first storage unit that stores a voltage based on theelectric current from the reference electric current source, a firstsupply unit that outputs an electric current generated by the voltagestored in the first storage unit, and a second storage unit that storesa voltage based on the electric current supplied from the first supplyunit; the second storage circuit includes a third storage unit thatstores a voltage based on the electric current from the referenceelectric current source, a second supply unit that outputs an electriccurrent generated by the voltage stored in the third storage unit, and afourth storage unit that stores a voltage based on the electric currentsupplied from the second supply unit, wherein the first electric currentsource supplies the first signal line with an electric current generatedby a voltage stored in the second storage unit; and the second electriccurrent source supplies the second signal line with an electric currentgenerated by a voltage stored in the fourth storage unit.

(13) In the image sensor as recited in (12), the first storage circuithas a first adjustment unit that is provided between the first supplyunit and the second storage unit and adjusts an electric current flowingfrom the reference electric current source to the second storage unitwhen the first readout circuit reads out the first signal to the firstsignal line to be smaller than that when the first readout circuit doesnot read out the first signal to the first signal line; and the secondstorage circuit has a second adjustment unit that is provided betweenthe second supply unit and the fourth storage unit and adjusts anelectric current flowing from the reference electric current source tothe fourth storage unit when the second readout circuit reads out thesecond signal to the second signal line to be smaller than that when thesecond readout circuit does not read out the second signal to the secondsignal line.

(14) In the image sensor as recited in (12) or (13), the first storagecircuit stores a voltage based on the electric current from thereference electric current source in the first storage unit when thefirst readout circuit reads out the first signal to the first signalline; the second storage circuit stores a voltage based on the electriccurrent from the reference electric current source in the fourth storageunit when the second readout circuit reads out the second signal to thesecond signal line.

(15) In the image sensor as recited in (12) to (14), the third storagecircuit stores a voltage based on the electric current from thereference electric current source after the voltage based on theelectric current from the reference electric current source is stored inthe first storage circuit.

(16) In the image sensor as recited in (9) to (15), the first electriccurrent source includes a first transistor having a first drain partconnected to the first signal line and a first gate part connected tothe first storage circuit and the first drain part; and the secondelectric current source includes a second transistor having a seconddrain part connected to the second signal line and a second gate partconnected to the second storage circuit and the second drain part.

(17) In the image sensor as recited in (12) to (15), the first storageunit and the second storage, the first supply unit, and the secondsupply unit are provided on a second semiconductor substrate differentfrom the first semiconductor substrate provided with the first readoutcircuit and the second readout circuit.

(18) In the image sensor as recited in (17), the first semiconductorsubstrate is stacked by the second semiconductor substrate.

(19) An image-capturing device comprising the image sensor as recited in(9) to (18).

The disclosure of the following priority application is hereinincorporated by reference:

-   Japanese Patent Application No. 2015-195279 (filed Sep. 30, 2015)

REFERENCE SIGNS LIST

3 . . . image sensor, 17 . . . signal line, 30 . . . electric currentsource, 100 . . . readout circuit, 110 . . . storage circuit

1. An image sensor comprising: a readout circuit that reads out a signal to a signal line, the signal being generated by an electric charge resulting from a photoelectric conversion; a holding circuit that holds a voltage based on an electric current from a power supply circuit; and an electric current source including a transistor having a drain part connected to the signal line and a gate part connected to the holding circuit and the drain part, the electric current source supplying the signal line with an electric current generated by the voltage held in the holding circuit. 